A conventional common in-vehicle wireless communication apparatus includes, for example, a radio or TV broadcast receiver, a GPS (Global Positioning System) receiver, and a wave beacon receiver for detecting the position of an own vehicle.
With development of a wireless communication technology in recent years, there is a growing tendency to provide a wireless communication function to even various products or uses with which the wireless communication function has been not equipped so far for the purpose of increasing an added value thereto. Such products and uses have been accepted by users, and further its range of application for the products continues to expand.
The aforementioned tendency is similar in an in-vehicle information device around a car navigation system, and user applications introducing a variety of wireless technologies have been provided.
For example, the following are given: a toll payment with ETC (Electric Toll Collection system) (Registered trademark; its indication will be hereinafter omitted); an in-vehicle wireless communication system for a communication connection between a user device and the in-vehicle information device, or between the in-vehicle information devices through Bluetooth (Registered trademark; its indication will be hereinafter omitted), a wireless LAN (Local Area Network), and/or the like; a connection between a mobile phone and an outer communication system with the wireless LAN or the like; and an in-vehicle dedicated wireless audio transfer system. Also, the user applications employing these wireless technologies are increasingly installed in the in-vehicle information device.
In the meantime, when there coexist wireless communication apparatuses that perform a wireless communication by different communication systems using close frequencies, there is a possibility of degradation in the communication quality due to radio frequency interference, in other words, radio waves used by the mutual wireless communication apparatuses turn to noise. In particular, frequency channels in a wireless LAN and Bluetooth equipped in many products such as mobile phones, notebook PCs, and game devices use frequencies in a 2.4 GHz frequency band. For this reason, the aforementioned radio frequency interference tends to occur.
Additionally, an arrangement of the frequency channels in the wireless LAN is decided as disclosed in Non-Patent Document 1, and an arrangement of the frequency channels in Bluetooth is decided as disclosed in Non-Patent Document 2. Note that the frequency channels to be used are decided by an operation of each country.
Also, the 2.4 GHz band is called an ISM (Industrial Scientific and Medical) band, and has a less stringent operation regulation as compared with other frequency bands; a radio station's license therefor is unnecessary, and further its use is wide.
Particularly, it is widely used in various energy saving wireless data communication systems, and also used in a domestic microwave oven, a medical device, and so on other than the communication application.
Typical wireless communication systems that use the frequency in the 2.4 GHz band are wireless LANs and Bluetooth. These wireless communication systems have their own interference avoidance functions.
For example, in the wireless LAN, by a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) function, whether or not a frequency band to be used by itself is being used by another wireless communication apparatus is determined by actually measuring a use condition of radio waves, and when a determination is made that the corresponding frequency band is not being used by another wireless communication apparatus, a communication thereof is carried out. In this way, radio wave interference and collision thereof with another wireless communication apparatus or a wireless communication apparatus of another wireless communication system are avoided.
It is noted that when another wireless LAN device or the wireless communication apparatus of another wireless communication system occupies the frequency band, a ratio occupied in the wireless communication of its own is reduced by that occupation, so that the transmission efficiency is lowered.
In Bluetooth, frequency hopping is carried out to perform spread spectrum communication, so that probability of radio wave interference in a specific frequency channel is lowered.
In addition, in Bluetooth, a communication state in each hoping frequency (hopping channel) is measured by using an AFH (Adaptive Frequency Hopping) function, the frequency in which the communication state gets worse due to radio wave interference and so on is removed from the hopping frequencies for non-use, so that the radio wave interference is avoided. However, since there is a lower limit in the number of the hopping frequencies, when a high percentage thereof in the use frequency band is subjected to the interference, the communication deterioration becomes larger.
Recently, a wireless communication apparatus in which wireless devices of different wireless communication systems of the wireless LAN, Bluetooth, and so on are equipped in one apparatus becomes widespread. In this wireless communication apparatus, when the wireless devices of respective communication systems are used simultaneously, the use frequency band is the same, and thus, deterioration of the communication quality occurs due to radio wave interference without performing appropriately allocation of the frequency channels or performing time sharing control even though the use frequency channels overlap each other.
As a conventional technique to avoid the aforementioned radio frequency interference, there are the following ones, for example:
In Patent Document 1, there is disclosed a composite wireless apparatus in which respective wireless devices of the wireless LAN and Bluetooth are equipped in one apparatus. In this apparatus, a control unit that controls the wireless channels of the two wireless devices performs control such that corresponding to the use frequency channels in the wireless LAN, the hopping frequencies in Bluetooth are not allocated to the frequency channels in the wireless LAN and these neighboring frequencies so as to be out of the carrier sense in the wireless LAN. In this manner, the radio frequency interference between the wireless devices equipped in the composite wireless apparatus is avoided.
In addition, in Patent Document 2, there is described an in-vehicle wireless communication apparatus in which respective wireless devices of the wireless LAN and Bluetooth are equipped in one apparatus. This apparatus includes: an application for a connection with a mobile terminal located in the vehicle through Bluetooth; an application for a connection with a wireless LAN station device located outside the vehicle through the wireless LAN; and a storage unit that stores the communication area of the wireless LAN station device outside the vehicle, and the following are predicted: an own vehicle enters this communication area, and a use frequency channel in the connection through the wireless LAN. Thus, allocations of the use frequency channels are optimized for the connection with the mobile terminal located in the vehicle through Bluetooth and the connection with the wireless LAN station device through the wireless LAN to thus avoid the mutual radio frequency interference between the wireless devices of Bluetooth and the wireless LAN.
However, even if the frequency channels between the wireless devices are separated by the optimized frequency channel allocations, when the mutual interference level becomes larger, the reception level of the wireless communication apparatus becomes excessive, so that the reception signals are deteriorated, which may result in a state of disabled reception.
To deal with such an inconvenience, in Patent Document 3, a switch is provided at a reception part of a mobile wireless device, and an intensity detection of a reception signal is performed by I and Q signals after frequency conversion and orthogonal decoding; corresponding to a level thereof, an attenuator upstream of a low-noise decoder is turned on and off, and a bypass route of a low-noise amplifier is turned on and off, to thus reduce an influence by disturbance waves, namely interference waves.
Further, in Patent Document 4, there is disclosed a wireless device with a gain switching antenna that makes an antenna gain variable such that an antenna provided with a plurality of attenuators are switched by a switch. This wireless device performs control such that reception areas of a plurality of remote wireless devices using the same frequency channel arranged in the same site do not overlap each other in such a manner that a transmission area of its own station is provided variably by switching the antenna gain, thereby avoiding mutual radio wave frequency interference.